Sensor chip and apparatus for tactile and/or flow sensing

ABSTRACT

A sensor chip, comprising a flexible, polymer-based substrate, and at least one microfabricated sensor disposed on the substrate and including a conductive element. The at least one sensor comprises at least one of a tactile sensor and a flow sensor. Other embodiments of the present invention include sensors and/or multi-modal sensor nodes.

PRIORITY CLAIM

The present application claims the benefit of U.S. ProvisionalApplication Ser. No. 60/476,672, filed Jun. 6, 2003, under 35 U.S.C.119.

STATEMENT OF GOVERNMENT INTEREST

The invention was made with Government assistance under NSF Grant Nos.IIS-00-80639 and IIS-99-84954, AFOSR Grant F49620-01-1-0496, and NASAGrant No. NAG5-878 1. The Government has certain rights in theinvention.

FIELD OF THE INVENTION

The invention concerns sensors. The invention relates generally to thefield of microscale sensors.

BACKGROUND OF THE INVENTION

Humans and other animals are able to perceive and process environmentalconditions using various sensory attributes. For example, animal skinand hair act to provide tactile and flow sensing for perception in landand/or water environments. Man-made devices rely on sensors constructedon many different physical principles, for example heat and resistance,to obtain similar information. Animal sensory systems have attributesthat are more elegant and efficient than known sensors.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a sensor chip comprising aflexible, polymer-based substrate and at least one microfabricatedsensor disposed on the substrate and including a conductive element. Theat least one sensor comprises at least one of a tactile sensor and aflow sensor. Other embodiments of the present invention are directed toparticular sensors and/or multi-modal sensor nodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary tactile sensor node incorporated into a sensorchip, according to a preferred embodiment of the present invention;

FIG. 2 shows a flexed sensor chip, according to a preferred embodimentof the present invention;

FIGS. 3A and 3B show a cross section of a hardness sensor, and thehardness sensor in contact with an object, respectively, according to apreferred embodiment of the present invention;

FIG. 4 shows a differential response between the membrane hardnesssensor and a reference sensor versus object hardness, with a linear fitline;

FIG. 5 shows a preferred embodiment of a thermal conductivity sensor,according to an embodiment of the present invention;

FIG. 6 shows a relationship between thermal conductivity and a timeconstant, in which step power input to a gold heater of a thermalconductivity sensor generates a signal at a nickel temperature sensor,with a time constant that varies with contact object thermalconductivity;

FIG. 7 shows a response of a skin mapping sensor to skin curvature,according to an embodiment of the present invention;

FIGS. 8A-8E show an exemplary process for manufacturing a sensor chiphaving sensors, according to a preferred embodiment of the presentinvention;

FIG. 9 shows a membrane hardness sensor with a nichrome string gauge,and a reference bulk sensor, respectively;

FIG. 10 shows an exemplary flow sensory node, according to anotherembodiment of the present invention;

FIG. 11 shows a silicon based artificial haircell, according to anembodiment of the present invention;

FIG. 12 shows a preferred artificial haircell (AHC), according to apreferred embodiment of the present invention;

FIGS. 13A and 13B show steps in a preferred process for manufacturingthe AHC of FIG. 12, according to a preferred embodiment of the presentinvention;

FIG. 14 illustrates a post release Ni plating set up, in which anexternal magnetic field is used to raise the AHC of FIG. 12, accordingto a preferred embodiment of the present invention;

FIGS. 15A and 15B show a plastically deformed Au hinge without and withelectroplating, respectively;

FIG. 16 shows an array of AHCs, having different heights and widths;

FIG. 17 shows resistance change versus deflection for an 850 μm long and200 μm wide cilium;

FIG. 18 shows airflow response of AHCs inside a wind tunnel, havingvarious cilium widths and lengths;

FIG. 19 shows a multidimensional array of AHCs;

FIG. 20 shows a three-dimensional array of hot wire anemometers,according to an embodiment of the present invention;

FIG. 21 shows a polymer membrane diaphragm supporting metal leads for apressure sensor and a shear stress sensor, according to a preferredembodiment of the present invention;

FIG. 22 shows an exemplary cluster of sensor nodes disposed about a dataprocessor;

FIGS. 23A-23C show methods for placing a data processor on a polymersubstrate;

FIG. 24 shows a flexible silicon chip; and

FIG. 25 shows steps in an exemplary process for forming an elastomerskin with embedded silicon islands.

DETAILED DESCRIPTION

For machines such as robotics to replace or serve as extensions ofhumans in dangerous, delicate, or remote applications, such machinesshould have sensory input at least comparable to human senses. One ofthe most important senses for performing varied complex and precisetasks autonomously or remotely is the sense of touch.

Human beings, for example, employ a flexible, robust sensory skin with adistributed architecture to achieve accurate object identification anddexterous manipulation. Tactile feedback from human skin provides amultitude of information, including force, temperature, hardness,texture, and thermal conductivity. However, conventionally, machineshave not had the sensing capability to provide an equivalent sense of“touch”.

Providing artificial tactile and/or flow sensors that provide richsensor data incurs significant challenges. For example, an optimalartificial sensor would provide multiple sensing modalities, mechanicalflexibility and robustness, efficient signal processing, and highdensity of integration with signal readout and electronics. Further, itwould be preferred that such an artificial sensor would be capable ofbeing manufactured with high efficiency and relatively low cost.

Artificial sensors have been created to provide force imaging andmeasurement. Such sensors have included silicon-based sensors, usingpiezoresistive or capacitive sensing, and polymer-based approaches thatuse piezoelectric polymer films for sensing. Others have combined someof the strengths of silicon with polymer-based devices, such as byembedding silicon sensing elements in polymer skins, or by coveringsilicon-based devices in a protective polymer layer. Other devices havebeen used to measure contact force and object thermal properties.

A fundamental difficulty faced in creating artificial sensors such as“sensing skins” is that the sensors in operation would directly contacta variety of objects and contaminants under any number of loadingconditions. As a result, devices that incorporate brittle sensingelements such as silicon-based diaphragms or piezoresistors, evenembedded in protective polymers, typically cannot be used as aninterface “skin” between a robotic manipulator and the manipulatedobject. Devices made with pressure-sensitive rubbers that can withstandcontact have been provided, but they require serial manual assembly andprovide limited independent sensing modes.

According to preferred embodiments of the present invention, anartificial sensor chip (or a large-area patch) is provided on apolymer-based substrate, forming a skin. Preferably, the sensor chip isflexible, providing a sensory skin that can be, for example, mounted oncurved or other non-flat surfaces easily and can withstand mechanicalflexure and movement.

The sensor chip incorporates one or more metal film sensors. Thisprovides many functional advantages and uses. The sensors preferably aredistributed in an array, such as a two-dimensional array, having highspatial density and integrated signal processing capabilities. Thesensor chip and sensor components thereon preferably are sufficientlyrobust to survive mechanical contact with an external harsh environment.

Unlike sensors in an integrated circuit chip that are packaged inenclosed environments, individual sensors according to a preferredsensor chip are exposed. Also, it is preferred that a frontal surface ofthe sensor chip be relatively smooth and free from mechanicalprotrusions, etch holes, exposed wiring, or other flaws and designcompromises that would allow environmental contamination or acceleratedwear and failure of the device.

In a preferred sensor chip, the distributed sensors are connected usingsignal processing circuitry that is distributed spatially and canaccommodate multiple streams of analog sensor output with minimalfootprint and power. Local, distributed signal amplification andanalog-digital conversion are preferred to preserve signal-to-noiseratio (before a signal is broadcasted through wire leads). Local signalprocessing avoids the routing bottleneck associated with long wireleads.

The density of integration of the sensors on a preferred sensor chip mayreach as high as, for example, 1-10/mm². The maximum density on apreferred chip may be determined not only by sensor sizes but also bythe footprint of signal processing circuits.

Also, in preferred methods of manufacturing the sensor chip, the cost ofmanufacture should be as low as possible to allow widespread use,especially if large continuous sensor chip surfaces are required.Manufacturing processes are preferably integrated and efficient.Particularly, monolithic integration is preferred because costs can bereduced through batch fabrication. It is also preferred that the effortsfor calibrating three-dimensional sensor positions should be minimizedto streamline their use.

Preferred sensor chips include multi-modal sensor nodes that are fortactile sensing and/or for flow sensing. For example, a multi-modaltactile sensing node may be provided.

A preferred multi-modal tactile sensor node can successfully incorporatemultiple sensor modalities for evaluating one or more of contact forces,and the relative hardness, thermal conductivity, and/or temperature of acontacted object.

Traditional microfabricated tactile sensors suffer from a number ofsignificant disadvantages. For example, they are typically based onsilicon, which is usually a rigid and fragile material from a mechanicalpoint of view. Exposing the sensors presents problems if silicon isused, because silicon is easy to fracture upon mechanical impact andover-loading. For example, many silicon micromachined tactile sensors donot stand force loading well.

The individual sensors of each multi-modal sensory node are fabricatedon the polymer-based substrate using surface micromachining. Thin-filmmetal elements are used, for example, as piezoresistors, heaters, andtemperature sensors. Preferred methods for manufacturing the individualsensors involve a relatively low temperature and do not involve bulkmicromachining. In this way, all of the sensors can be formed on thepolymer-based substrate.

Also, traditional silicon sensors only sense surface roughness featuresand contact forces. By contrast, a preferred tactile sensing node maycontain one or more of surface roughness, contact force measurement,thermal conductivity, hardness, temperature, and/or proximity sensors.Such additional modalities preferably allow a preferred tactile sensornode to characterize an object in a more comprehensive fashion.

Another exemplary multi-modal sensor node that may be formed on asurface of the sensor chip is a flow sensor node. A preferredmulti-modal flow sensor node can characterize a boundary-layer flowfield in a comprehensive fashion, with high spatial and temporalresolution. Such exemplary multi-modal flow sensor nodes may be used,for example, in real-time monitoring of a flow field in underwatervehicles and structures, and in characterizing flow fields around modelsin experimental wind or water tunnels.

Traditional flow sensors are based on hot-wire anemometry for measuringflow speed, or diaphragms for measuring pressure distribution. Suchdifferent sensors typically have been based on specific structures thatare significantly incompatible with fabrication processes and materials.Accordingly, it has been impossible to measure several flow parameterslocally and with a distributed array. By contrast, a preferredmulti-modal flow sensor node includes one or more of various flowsensors, including, for example, surface micromachined artificialhaircell sensors (for flow rate), surface micromachined hot-wireanemometers (for flow speed distribution, preferably along three axes),and surface micromachined diaphragms, preferably manufactured fromParylene, for pressure sensors and shear stress sensors (for vortex anddrag detection).

A preferred sensor chip substrate is manufactured primarily frompolymer-based materials, as opposed to silicon. Because silicon is arelatively fragile material for sensors, sensor chips made out ofpolymer material offer desirable mechanical flexibility and robustnesscompared with silicon counterparts. However, most existing polymermaterials such as silicone elastomer, polyimide, and plastics cannothost signal processing electronics like silicon substrates do.

Hence, a preferred sensor chip integrates flexible polymer devices withdiscrete silicon chips for signal processing. The silicon chips(islands) are selected and designed so as not to significantly impedethe overall mechanical flexibility and surface integrity of the sensorchip, and so that they can be integrated in efficient manufacturingprocesses without significantly compromising cost.

Exemplary applications of a preferred sensor chip include, but are notlimited to, smart tactile skins for sensor-rich surgical tools, roboticsmanipulators, computer periphery input devices, and smart toys havingsensor input. Preferred sensor chips having flow sensors may be usefulfor, e.g., smart flow sensing skins for underwater robots (e.g., forexploration or mine detection), underwater vehicles and infrastructures(e.g., oil drilling stations in deep sea), and scientific explorationand measurement (e.g., wind tunnels). Preferred embodiments of thesensory chip have the potential to make a significant impact on a broadrange of applications for industry, exploration, military, and security,as nonlimiting examples.

Referring now to FIG. 1, an exemplary sensor chip 10 is shown, embodiedin a flexible polymer-based substrate 12 forming a skin, and including aplurality of multi-modal sensor nodes 14, shown as multi-modal tactilesensor nodes. As shown in FIG. 1, the multi-modal sensor nodes 14 arerepeated over an n×n array (as shown, 3×3) to form the sensor chip 10. Apreferred multimodal tactile sensor node, for example, includes multiplesensor modalities (hardness, thermal conductivity, temperature, contactforce, surface roughness). These nodes 14 in an exemplary embodiment arerepeated with a spatial frequency of approximately 1 per 1 cm², thoughthis repetition or particular distribution is not necessary. Forexample, individual nodes may have the same number of sensors or asignificantly different number and/or type of sensor. Also, the spatialfrequency of the nodes can vary, and may be greater or fewer than 1 per1 cm².

The multi-modal sensor node 14, a tactile sensor node, includes atemperature sensor 16, a thermal conductivity (thermal flux) sensor 18,and a contact force and measured hardness sensor 20. The multi-modalsensor node 14 also includes a reference hardness sensor 22 for use withthe contact force and measured hardness sensor 20. Sensors may also beimplemented for such tasks as object identification and impendingslippage detection. In the preferred tactile sensor node 14, a referencenickel resistance temperature device (RTD) of the temperature sensor 16provides temperature measurement and compensation, a gold heater 24 andnickel RTD 26 pair provides thermal conductivity measurement for thethermal conductivity sensor 18, and the membrane NiCr (nichrome)strain-gauge based contact force and hardness sensor 20 with thereference contact hardness sensor 22 measures hardness.

The substrate 12 is preferably made of a polymer-based material. In anexemplary sensor chip 10, the substrate is a 2 mil thick Kapton HN200polyimide film, manufactured by E.I. DuPont de Nemours and Co. Thepolymer substrate allows flexibility, robustness, and low material cost.Flex channels 30 are provided in the substrate along two dimensions byforming indentations in the substrate 12. The flex channels 30 provideenhanced and controlled flexibility to the substrate 12.

In addition, the contour of the substrate 12 is sensed in an integratedfashion using mapping sensors 32 embodied in microscale strain gauges,also preferably made of NiCr, and dispersed between the sensory nodes 14(as shown in FIG. 1, the tactile sensor node). The mapping sensors 32are dispersed between the sensory nodes 14 to sense bending of thesubstrate. In this way, the contour of a bent skin is sensed in anintegrated fashion using the mapping sensors 32. For example, when thesensor chip 10 is mounted on a curved or compliant surface (e.g., arobotic finger tip), as shown by example in FIG. 2, the spatial relationof the multi-modal sensor nodes 14 is mapped to coordinate manipulationin three-dimensional space.

Individual sensing elements will now be described in more detail. Asshown in FIG. 1, a preferred temperature sensor 16, for example,includes a nickel resistance temperature device (RTD) 34 that is used tomeasure the temperature of the operating environment as well as contactobjects. This information is important for temperature compensation ofthe measurements of the other sensors as well as providing contactobject information. The temperature sensor 16 and other sensorcomponents are connected to other parts of the sensor chip, such as aprocessor, by leads 36.

Because all the sensors 16, 18, 20, 22 incorporated on the exemplarytactile sensing node are based on thin film metal resistors, all of themwill function as RTDs to one extent or another based on the TCR (thermalcoefficient of resistance) of the base material. This value is low forNiCr, making it a good choice for rejecting thermal disturbances, but ishigh for nickel and gold. Gold is not used for a preferred RTD due toits low resistivity. By using nickel, a high TCR is provided with theadded benefit of increased resistivity to decrease the effect ofparasitic resistances. The TCR of each sensor is characterized to allowtemperature compensation by calibrating the reference nickel RTD, forexample, by heating the sensor chip 10 and observing the changes inresistance with temperature, then calculating the base metal TCR.

Hardness of a contact object is an important parameter for objectidenfication and manipulation. This measurement modality is lacking inmost conventional tactile sensors. Existing micromachined hardnesssensors require that the applied force be known, use a known calibratedintegral actuator force, or use changing resonant frequency underultrasonic vibration. The required assumptions, complexity, and sizelimitations of such approaches do not lend themselves to a distributedmulti-modal sensor chip. By contrast, a preferred hardness sensor 40shown in FIGS. 3A-3B is a passive hardness sensor that does not rely onactuation or knowledge of contact force.

Referring now to FIGS. 3A-3B, the passive hardness sensor 40, which maybe incorporated into the multi-modal sensory node 14, derives a hardnessof a contact object using two contact sensors of different supportstiffness: the contact force and measured hardness sensor 20 and thereference hardness sensor 22. The preferred hardness sensor does notrely on knowledge of contact force. In a preferred embodiment, themeasurement sensor 20 is mounted on a polymer membrane, while thereference sensor 22 is built on the bulk substrate 12. Both themeasurement sensor 20 and the reference sensor 22 include a strain gauge42, which may be made from NiCr, for example, to measure response. Adifferential response between the measurement sensor 20 and thereference sensor 22 is used to measure the hardness of a contact object43.

The structure of the preferred hardness sensor 40 within the sensor node14 is shown in FIG. 3A and in cross section in FIG. 3B. The exemplaryhardness sensor 40 includes the measurement sensor 20 on a squarepolymer diaphragm 45 and a reference sensor 22 on the bulk polymersubstrate 12. Both sensors 20, 22 include a contact mesa 46 with thestrain gauges 42 situated on the periphery of these mesas. The square ofa diaphragm 45 of the measurement sensor 20 has a relatively lowstiffness and for a given maximum central displacement requires auniform pressure according to clamped-clamped plate theory as shown inEq. 1.

$\begin{matrix}{q_{plate} = \frac{z_{\max}{Et}^{3}}{(0.0138)b^{4}}} & (1)\end{matrix}$

In Eq. 1, z_(max) is the peak vertical deflection in the center of thediaphragm 45, q_(plate) is the pressure applied to the plate, b is thelength of the square sides, E is the material modulus, and t is theplate thickness.

The preferred reference sensor 22 does not use a diaphragm; rather thecontact mesa 44 and the strain gauges 42 are positioned over fullthickness bulk polymer 12. The stiffness of the bulk reference sensor 22is thus much higher than the measurement sensor diaphragm 45. Thepreferred reference sensor 22 requires a uniform pressure for a givendeflection according to Eq. 2.

$\begin{matrix}{q_{bulk} = \frac{z_{\max}E}{(2.24){a\left( {1 - v^{2}} \right)}}} & (2)\end{matrix}$

In Eq. 2, ^(θ)is the bulk material Poisson's ratio, a is the contactmesa 46 width, and q_(bulk) is the pressure applied to the bulk sensorcontact mesa. This model assumes that the reference sensor 22 behaveslike a semi-infinite block under a uniform pressure over the area of thecontact mesa.

When the sensor chip 10 is in contact with the object 43, changes inresistance are observed at both the measurement and reference sensorstrain gauges 42. The measured resistance changes are converted to apeak deflection (z_(max)) with calibrated resistance versus displacementdata and used to find the apparent pressures q_(plate) and q_(bulk) withEqs. 1 and 2. The contact object hardness 43 is related to the ratio ofapparent pressures.

Measurement of contact forces can also be performed using themeasurement sensor 20 and the reference sensor 22. Based on the knowngeometry of the devices, the pressures can be equated to normal force.The differential stiffness of the two sensors 20, 22 allows twodifferent ranges of contact forces to be measured.

In an experimental operation of the hardness sensor 40, a number ofpolymer samples were placed in contact with the sensor skin 12. A rangeof reference samples of sorbothane and polyurethane rubber with knownhardnesses ranging from 10 to 80 Shore A were cut into 5 mm by 5 mmsquares and pressed onto the sensor skin 12 using a fixed mass (147 g).The change in resistance of each sensor 20, 22 was converted to anequivalent displacement using calibration data. Calibration data wasgenerated by measuring the change in resistance of the measurementmembrane sensor 20 and the bulk reference sensor 22 in response to aknown normal displacement provided by a micromanipulator probe coupledto a precision linearly variable differential transformer (LVDT).

The proportionality between pressure ratio and object hardness is shownin the graph of FIG. 4. A large amount of scaffer was observed in thehardness data as can be seen in the graph. This is attributable to thesurface roughness of the rubber samples. Nevertheless, a clear overalltrend is observed when a large number of data points are averaged as inFIG. 4, showing an increase in pressure ratio with object hardness.

The thermal conductivity of the contact object 43 is another importantpiece of data for object identification. The thermal conductivity sensor18 operates by observing the changing resistance of the nickel RTD 26 inresponse to an input to the gold heater 24. The thermal conductivity ofthe contacting object 43 is a useful measure for object discrimination,and in concert with other sensing modes can expand the capabilities ofthe overall sensor chip 12 by helping to distinguish between equally“hard” objects for example.

As shown, the value is derived by measuring heat flux between the heater24 and the temperature sensor 26, which are disposed on the polyimidesubstrate 12. The heater 24, preferably manufactured from gold asdescribed above, is disposed on a bump 48 (FIG. 5) formed on thesubstrate 12, and is situated near, yet separated from, the temperaturesensor 26. The temperature sensor 26 is embodied in an Ni RTDthermoresistor, also disposed on the bump 48. The heat transfer betweenthe heater 24 and the temperature sensor 26 is altered when the contactobject 43 contacts the surface of the sensor chip 12 over the thermalconductivity sensor 18, which changes the thermal transfer path. Theheat flux travels through the contact object 43 as well as the substrate12, which changes the signal measured at the temperature sensor 26. Astepped power input to the heater generates a signal at the temperaturesensor with a time constant that varies with the thermal conductivity ofthe contact object.

When not in contact with the object 43, the only route for the heatinput of the heater 24 to reach the RTD of the temperature sensor 26 isthrough the polyimide substrate 12 and the surrounding air. When theobject 43 comes in contact with the thermal conductivity sensor 18, thelow efficiency heat path through the air is replaced by solidconduction, changing the character of the signal measured at thetemperature sensor 26. Using an Ni RTD as the temperature sensor 26, forexample, with a square wave voltage input to the heater, the temperatureof the temperature sensor can be modeled as a simple first order systemaccording to Eq. 3.T _(RTD)(t)=1−e ^(−1/τ)  (3)

Where τ is the time constant of the first order system, giving a measureof how quickly the system responds to an input. The time constant of thetemperature of the temperature sensor 26 is found to be a function ofcontact object thermal conductivity. This method was found to correlatewell to contact object thermal conductivity.

In an exemplary operation, characterization of the performance of thethermal conductivity sensor is performed at room temperature (˜22° C.)by inputting a 0-2 VDC square wave at 0.3 Hz to the gold heater 24 andmeasuring the resulting change in resistance of the nearby Ni RTD 26.The resistance of the RTD is sampled at 10 Hz using an Agilent 33410Amulti-meter and GPIB interface.

The thermal conductivity sensor 18 preferably should behave as a firstorder system with a time constant related to the object thermalconductivity. FIG. 6 shows the result of testing, where contact objectsof various thermal conductivities (nylon 6, soda-lime glass, singlecrystal silicon, 300-series stainless steel, aluminum, and ambient air)were placed in contact with the surface of the thermal conductivitysensor 18, and the time constant of the resulting signal at thetemperature sensor 26 was obtained through curve fitting. It wasobserved that the time constant decreases and the step response of thetemperature of the temperature sensor 26 is faster with increasingthermal conductivity. Scatter is observed and expected due to changes incontact configuration from test to test due to surface roughness. Therelationship between object thermal conductivity and time constant isfound to be approximately logarithmic based on a curve fit of FIG. 6. Asshown, more conductive objects result in faster response and smallertime constant.

Another type of sensing measures curvature of the substrate using themapping sensor 32 described above. The mapping sensor 32 preferablyembodied in integrated NiCr strain gauges dispersed between the sensornodes 14 measures the x- and y-direction curvature of the flexiblesubstrate 12. The mapping sensors 32 are positioned over the flexchannels (trenches) 30 etched in the back of the polyimide substrate 12to allow the substrate to preferentially bend in these regions.Processing of these measurements into bending angles using calibrateddata allows a three-dimensional mapping of skin curvature state. Theskin mapping sensors 32 are found to perform linearly (R²=0.996) withrespect to curvature with sensitivity of 44.25 ppm.

Skin curvature calibration is accomplished by flexing the substrate 12under known displacement using a micromanipulator coupled to a precisionlinearly variable differential transformer (LVDT). Measurements aretaken while bending and relaxing to assess visco-elastic hysteresis andplastic deformation. A resulting response of the mapping sensors 32versus skin flex for a number of tests is seen in FIG. 7.

The processing steps preferably do not have to involve high temperaturesteps or bulk micromachining, therefore they can be substrate neutral.Specifically, the microfabrication process can be carried out directlyon flexible and low cost polymer substrates.

A description of an exemplary fabrication process follows for thesensory chip and the tactile sensory node, referring to FIGS. 8A-8E. Apolyimide film substrate 60, for example a 50 mm square sheet cut from asheet of DuPont Kapton HN200 polyimide film is provided. This film 60 ispreferably about 50 μm thick, though other thicknesses may be used.During the fabrication of the polyimide film 60, one surface of the filmis in contact with a roller and the other is untouched. In practice,measurements with an optical vertical scanning interferometer (VEECOLM1000) showed very small roughness differences between the free androller faces (197 nm and 243 nm Rq respectively). Prior tophotolithography, the polyimide film substrate 60 is cleaned and thenbaked at 350° C. under nitrogen at 1 Torr for 2 h.

Once the polyimide film substrate 60 has been cured, an aluminum etchmask 62 is deposited and patterned via lift off on the “rough” rollerside of the film (FIG. 8A). The film substrate 60 is then etched in anoxygen plasma reactive ion etcher at 350 W with 300 mT oxygen pressure(FIG. 8B) to define the flex channels 30 and the membrane sensordiaphragms 45. The film 60 preferably is etched 40 μm down at a rate of˜330 nm per minute. This plasma-etching step preferably is performedfirst to avoid erosion of backside metal layers that may otherwiseoccur.

With the sensor node 14 regions and contact force membranes defined, a2-μm-thick layer of photo-definable polyimide (for example, HDMicrosystems HD4000) is spun on the smoother top skin surface andpatterned to define contact mesas 46 for the thermal conductivity 18 andreference RTD sensors 22 (FIG. 8C). FIGS. 9A-9B show exemplary RTDstrain gauges on a membrane hardness sensor 20 and a reference bulksensor 20, respectively. This layer is aligned to the backside featuresvia alignment marks visible due to the optical clarity of the HN200film. Once patterned, the polyimide layer is cured under 1 Torr ofnitrogen at 350° C. for 2 hours.

Next, nickel RTDs 26 are patterned and deposited on the contact mesas46. For example, nickel (e.g., 500Å Ni) 64 is thermally evaporated viae-beam on top of a 100 A chrome adhesion layer (not shown). Then, 750 Åof NiCr 68 is deposited and lifted off to define the strain gauges forthe force 20, curvature (mapping) 32, and hardness sensors 22.Preferably, no adhesion layer is used. In order to achieve therelatively high resolution required for the minimum NiCr (10 μm) and Ni(15 μm) feature widths on a flexible polymer substrate, in a preferredembodiment, the Kapton film 12 is temporarily attached to a Pyrexsubstrate via surface tension by wetting the substrate with a drop ofde-ionized water. The RTDs 26 are patterned preferably via liftoff usingstandard image reversal photolithography. The last metal layer comprises1500A of gold 70 on a 100 Å chrome adhesion layer that is thermallyevaporated and lifted off (FIG. 8 D), forming wiring 36. Before eachmetal deposition step 60, the film substrate is placed in oxygen planarplasma for 3 minutes at 300W to remove photoresist residue from imagereversal and to improve metal adhesion to the polymer film.

The final step is to spin on and pattern the tactile contact bumps 46for the force and hardness sensors (FIG. 8E). The bumps 46 are definedfrom an 8-μm-thick layer of HD4000 photo-definable polyimide in thecenter of each sensor. The polyimide is cured for 2 hours at 350° C. and1 Torr nitrogen.

Another embodiment of the present invention includes a sensor node forflow sensing. The substrate may be polymer-based as in the substratesupporting the tactile sensor node 14.

Comprehensive flow sensing in the fluid boundary layer involvesmeasurement of, for example, pressure, shear stress (drag and vortex),temperature, and three-axis flow rates. The spatial and temporalevolution of surface flow features is extremely difficult to obtain dueto limitations of scientific instruments.

Conventional flow sensing instruments such as hot-wire anemometers aresingular point measurement devices only. They suffer from a number ofbottlenecks: their sizes are large and may change the characteristics ofthe flow; it is extremely difficult to measure multiple flow parametersincluding vector speed, pressure, and shear stress, which isproportional to the gradient of velocity in the boundary layer; and itis difficult to characterize a flow field within a thin boundary layer(thickness on the order of 1 mm).

Microfabricated flow sensing surfaces with multiple sensing modalitiesto record pressure, shear stress, and flow rates would be useful forexperimental fluid mechanical studies and for underwater vehicles andplatforms. Such sensors preferably would be fabricated using efficient,low cost techniques. They preferably would allow integration ofmicroelectronics signal processing units, and should be relativelymechanically robust.

Potential application scenarios for multi-modal flow sensors mayinclude, but are not limited to: comprehensive monitoring of liquid flowfield for underwater vehicles and structures, such as autonomousunderwater vehicles, deep-sea drilling stations, and military vehiclesfor possible drag reduction; and comprehensive monitoring of air flowconditions for aircrafts and unmanned vehicles.

A large sensitive skin could be used to cover an object with a largearea and curved surfaces. For example, an aerodynamic model used in anexperimental wind- or water-tunnel may be covered with the sensitiveskin in strategic regions to provide direct experimentalcharacterization of flow field. Such flow field data has beenprohibitively difficult to obtain in the past. Such comprehensiveresults can be used to validate and improve theoretical models orprovide aerodynamic design insights.

The diagram of an exemplary single sensor node is illustrated in FIG.10. According to an embodiment of the present invention, the node 70,which may be disposed on a flexible, polymer-based substrate 72, isprovided with one or more of the following sensing units: an artificialhaircell 74 for measuring three-dimensional flow velocity, one or morehot-wire anemometers 76 along one or more dimensions for measuring thevelocity of flow at different distance to the boundary layer, a pressuresensor 78 for monitoring pressure variation, and a shear stress sensor80 for measuring surface vortex. Various sensors may be integratedtogether on the polymer substrate using novel material and fabricationprocesses as described herein.

Fish and many underwater animals utilize multimodal sensitive skin thatcan detect flow, pressure distribution, electrical potential and field,and local vortex. The lateral line is a primary sensing organ for fish.It usually spans the length of the fish body. Its main functions include(1) detection of water flow around the fish body, allowing a fish tomaintain stability within turbulent currents and (2) detection ofdistant objects such as obstacles, prey and predators using direct orreflected waves. Linearly distributed along the lateral line areclustered haircell bundles embedded in a gel-like dome called aneuromast. Water flowing past the neuromasts imparts forces to thehaircells and causes them to bend, with the extent of the bendingdetermined by the speed of the flow. In certain species, the haircellslie outside of the epidermis; in others, they are embedded in sub-dermalcanals for added protection against wearing and damages.

Artificial haircell sensors may be used for mimicking the lateral linesystem of fish. A schematic diagram of an exemplary haircell sensor 82,made of single crystal silicon substrate 83, is shown in FIG. 11. Thehaircell sensor 82 consists of an in-plane fixed-free cantilever 84 witha vertical artificial cilium 86 attached at the distal, free end.External flow parallel to the sensor substrate 83 impacts upon thevertical cilium 86. Due to rigid connection between the in-planecantilever 84 and the vertical cilium 86, a mechanical bending moment istransferred to the horizontal cantilever beam, inducing strain at thebase of the cantilever beam, which is detected using a strain sensor 88,such as a piezoelectric sensor producing a signal that is transmitted byconductive contacts 90. The magnitude of the induced strain can besensed by many means, for example by using integrated piezoresistivesensors.

The vertical cilium 86 preferably is realized using a three-dimensionalassembly technique called plastic deformation magnetic assembly (PDMA).A description of the PDMA process is provided in J. Zou, J. Chen, C.Liu, and J. Schutt-Aine, “Plastic Deformation Magnetic Assembly (PDMA)of Out-of-Plane Microstructures: Technology and Application; IEEE/ASMEJ. of Microelectromechanical Systems, Vol. 10, No. 2, pp. 302-309, June2001, which is incorporated in its entirety by reference. A preferredassembly process allows reliable formation of three-dimensionalstructures in large array format. Multiple structures can be achieved atwafer-scale by a globally applied magnetic field. Position and height ofthe cilia can be controlled.

A preferred polymer based haircell device is also provided, as shown byexample in FIG. 12. An artificial haircell (AHC) 92 includes a verticalbeam 94 (cilium) rigidly attached to the substrate 92. The verticalcilium 94 is made of surface micromachined polymer, and more preferablyincludes a stiff permalloy plating.

As shown in FIG. 12, the vertical cilium 94 is rigidly attached to thesubstrate 72 by one or more rigid metal supports 95. The substrate 72can be any of various substrates, but preferably is a polymer-basedsubstrate. Attached at the base of the vertical cilium 94, between thecilium and the substrate, is a strain gauge 96. The strain gauge 96includes a thin film nichrome (NiCr) resistor on a thicker polyimidebacking that runs the length of the cilium 94. The piezoresistive strainsensors 96 are located on the piece that is assembled (i.e., thevertical cilium 94 ) using three-dimensional assembly.

When an external force is applied to the vertical cilium 94, eitherthrough direct contact with another object (functioning as a tactilesensor) or by the drag force from fluid flow (flow sensing), the beamwill deflect and cause the strain gauge 96 to stretch or compress. Thestrain gauge region is treated as being rigidly attached to thesubstrate 72, while the cilium 94 is free. The magnitude of the inducedstrain (e) is largest at the base, where the strain gauge is located,

$\begin{matrix}{ɛ = \frac{M\; t_{PI}}{2{EI}}} & (4)\end{matrix}$

where M is the moment experienced at the base, t_(PI) is the polyimidethickness, and E and I are the modulus of elasticity of and the momentof inertia of the polyimide. The very thin nichrome resistor of thestrain gauge 96 is not taken into account.

The vertical cilium preferably is surface micromachined and deflectedout of plane using magnetic 3D assembly, such as PDMA, and can beconducted on a wafer scale. The vertical cilium 94 remains in deflectedposition due to plastic deformation at the joint.

A preferred fabrication method includes a series of metallization andpolymer deposition steps. Referring to FIG. 13A, first, on a substrate100 a 0.5-μm Al sacrificial layer 102 is evaporated and patterned. Then,a 5.5-μm photodefinable polyimide 104 (e.g., HD-4000 from HDMicrosystems) is spun-on and patterned photolithographically. Thepolyimide 104 is cured at 350° C. in a 1 Torr N₂ vacuum for 2 hours.Preferably, this is the highest temperature used in the process,allowing the AHC to be fabricated on various substrates 100, includingpolymer-based substrates.

Afterwards, a 750-Å-thick NiCr layer 106 used for the strain gauge 96 isdeposited by electron beam evaporation. This is followed by a0.5-μm-thick Au/Cr evaporation 108 used for electrical leads 110 and thebending hinge. The Au/Cr layer 108 is then used as a seed layer toelectroplate approximately 5 μm of permalloy 112 before being removed bylift-off. The resulting structure is shown in FIG. 13A. The finalsurface micromachining step is another 2.7-μm polyimide film (not shown)to serve as a protective coating for the permalloy cilium and the NiCrstrain gauge.

The Al sacrificial layer 102 is then etched in a TMAH solution for overa day to free the structure. The sample is then carefully rinsed andplaced in an electroplating bath 113, where an external magnetic fieldis applied that interacts with the permalloy 112 to raise the verticalcilium 94 out of plane.

For example, in a post-release Ni plating setup, shown by example inFIG. 14, an external magnetic field 114 is applied with an electromagnet115 during the electroplating process. Preferably, the entire process isdone under a microscope. After a few minutes of plating, the magneticfield 114 is removed and the cilium remains permanently out of plane.

While the external field is being applied, Ni 116 is electroplated onthe Au hinge using a nickel anode 118, which rigidly fixes the structureout-of-plane to the substrate and reinforces the ductile Au hinge, asshown in FIG. 13B. The Ni electroplating is done on the substrateglobally, preferably lasting about 20 minutes to achieve a thickness ofapproximately 10 mm. The actual thickness is difficult to measure andcontrol, but is not important as long as it is rigid relative to thepolyimide film.

SEM images of the hinge are shown in FIGS. 15A-15B, showing thedifference between a deformed Au hinge with and without Ni plating. Anarray of AHCs 92 with different vertical cilium and strain gaugegeometry is shown in FIG. 16, showing the parallel nature of thepreferred fabrication process. Again, it is preferred that overall, thefabrication method does not exceed temperature over 350° Celsius,allowing it to be completed on a skin-like thin film polymer substrateon other substrates. Silicon, glass, and Kapton film, for example, canbe used as a substrate for this process. The resistance of devicestested ranges from 1.2 kW to 3.2 kW, and TCR measurement of theas-deposited NiCr film in an exemplary AHC has a value of −25 ppm/° C.,which is very small and should not contribute to anemometric effectsduring airflow testing.

In an exemplary operation of the AHC 92, the resistance change due toexternal displacement is shown in FIG. 17 for an 850 μm tall verticalcilium. A micromanipulator is used to deflect the distal end of thevertical cilium. The resistance change is measured by a multimeter, andis linear to the beam deflection. The gauge factor GF can be calculatedfrom the slope of the curve,

$\begin{matrix}{{GF} = \frac{{dR}/R}{ɛ_{PI}}} & (5)\end{matrix}$

where dR/R is the percent resistance change, and e_(PI) is thecalculated strain from a fixed-free beam (See Eq. (4)) undergoing adeflection x. The plastically deformed hinge, after being plated withapproximately 10 μm of Ni, is very rigid. The modulus of elasticity forthe nickel is approximately two orders of magnitude larger thanpolyimide (200 Gpa versus 3.5 Gpa). Therefore, an assumption of afixed-free cantilever model should be valid. The measured gauge factorfor an exemplary strain gauge configuration is about 1.4, which is lowerthan expected. This could be attributed to the strain gauge not beinglocated at the point of maximum strain.

Several fabricated AHCs were then tested as airflow transducers in awind tunnel. The airflow with velocity U impinging on the cilium resultsin a drag force acting normal to the paddle, leading to a moment on thestrain gauge

$\begin{matrix}{M = {\int_{o}^{l}{C_{D}\frac{1}{2}\rho\; U^{2}{wy}{\mathbb{d}y}}}} & (6)\end{matrix}$

where C_(D) is the drag coefficient, r is the density of air, w and lare the width and length of the cilium. Because strain is proportionalto the applied moment, and resistance change is proportional to strain,Equation (6) suggests a quadratic relationship between airflow andresistance change. In addition, by systematically varying the height andwidth of the cilium, the response can be tailored to different ranges ofair velocity. The polarity of resistance change is dependant on thedirection of the airflow.

The wind tunnel measurement of three AHCs with different cilia geometryis plotted in FIG. 18. The AHCs tested were fabricated on a siliconsubstrate to allow wire bonding to the sample. The AHC with the longestcilium length of 1500 μm is the most sensitive, with dR/R reaching 600ppm at around 10 m/s. The device with the shortest cilium, even with agreater width, does not have the 600 ppm resistance change until 30 m/s.The sign of resistance change can be indicative of the direction of airvelocity. However, the response in various directions does not seem tobe symmetrical. This is because it is difficult for the PDMA assemblyprocess to orient the cilium at exactly 90° to the substrate. Thecharacteristic lengths of individual MEMS devices range from 1 μm to 1mm, although distributed microsystems containing arrays of devices couldhave larger overall sizes.

The artificial haircell, for example, may be used to realize othersensing modalities, including but not limited to vibration sensing. Byvarying the geometry and mass of the vertical cilium, the haircell canbe made more responsive to inertia forces created by vibration. Forexample, a three-axis acceleration sensor may be provided, as shown byexample in FIG. 19.

Among other flow sensor components, the hot-wire sensor 76 uses anelectrical wire placed in the flow field. The wire is heated using ohmicheating and the resistance of the wire (which is a function oftemperature) is monitored. Flow imparts forced convection on the wire toinduce cooling. The temperature of the wire indicates the flow speed.

Existing hot-wire sensors are all supplied as individual devices. Theirsizes are relatively large. Even micromachined hot-wire anemometers aresupplied as singular units. They cannot measure the distribution of flowin a distributed field. By contrast, a hot-wire sensor can be made usingsurface micromachining process and three-dimensional assembly method. Itcan be made on polymer substrates with large two-dimensional arrayformats. Examples of hot wire anemometers formed on a substrate andfabrication methods for them are provided in J. Chen and C. Liu,“Development and Characterization of Surface Micromachined, Out-of-PlaneHot-Wire Anemometer,” in Journal of Microelectomechanical Systems, Vol.12, No. 6, December 2003, pp. 979-988, and in J. Chen, J. Zou, and C.Liu, “A Surface Micromachined, Out-of-Plane Anemometer,” in ProceedingsMEMS, Las Vegas, 2002, pp. 332-335, which are incorporated by referencein its entirety herein. FIG. 20 shows a three-dimensional array ofhot-wire anemometers, which can be formed by selecting fabricatingindividual anemometers and raising them out of plane.

Conventional pressure and shear stress sensors employ a membrane. In thecase of a pressure sensor, the diaphragm bends in response to appliedpressure difference. In the case of shear stress for measuring fluidstress, the membrane supports a heated hot-wire element. Referring toFIG. 10, the pressure sensor 78 may include, for example, an NiCr straingauge 120 disposed on a Parylene film 122 forming a raised diaphragm formeasuring deflection of the Parylene film in response to pressure. Theshear stress sensor 80 may include a raised Parylene membrane with aheated hot-wire element such as a nickel thermoresistor 126 formeasuring fluid stress.

According to another embodiment of the present invention, amicrofabrication sequence for a Parylene membrane, shown by example inFIG. 21, with patterned metal on the membrane is provided, in which apreferably polymer membrane diaphragm supports metal leads used for apressure sensor and for a shear stress sensor. The metal leads can beused for both pressure sensing and shear sensing (temperature sensing).The location preferably determines the principal use of a particularmetal lead. For example, the metal leads closer to the center of themembrane may be better located for shear sensing, while the metal leadscloser to the edge of the membrane may be better located for pressuresensing.

In an exemplary fabrication process, a photoresist layer is depositedand patterned as a sacrificial layer to define a membrane cavity. Alayer of Parylene is deposited, preferably having a thickness in the 0.2to 5 μm range. A metal thin film is deposited and patterned to form aresistor that can respond to stress (piezoresistor). The gauge factor ofsuch resistors is typically approximately 1-5. Metals that can be usedinclude NiCr (nichrome), Pt, Au, Cu, Al, and others.

Another layer of Parylene is deposited on top of the metal thin film,passivating the resistors and reducing or preventing damage byenvironmental elements over the long run. The photoresist is removedthrough spatially placed holes on or around the membrane. The cavity isdried and sealed using one or more of a variety of methods. Oneexemplary method to seal the cavity is to deposit another thin layer ofParylene. The deposition process is performed at low pressure (e.g., 40mtorr), and the cavity is therefore sealed under low pressure.

In another embodiment of the present invention, exemplary methods areprovided for integrating silicon chips (containing signal processingfunctions such as amplification, multiplexing, and analog-to-digitalconversion) with a polymer sensor chip (with tactile or flow sensingcomponents) and within the fabrication flow. FIG. 22 shows an overviewof a skin architecture showing a cluster of sensor nodes connected to alocal cluster processor.

A first method includes bonding a silicon chip, such as a commerciallyobtained chip 130 (e.g., ADC chip with internal clock from NationalSemiconductors) onto a polymer sensor skin 132. The chip may be, forexample, an application-specific IC chip. A schematic diagram of thisbonding approach is shown in FIG. 23A. In a preferred bonding process, ablank slot 134 on the back surface of the sensor skin 132 is opened forthe microelectronic chip 130 to rest. A through-wafer electricalinterconnect 138 is provided so that the silicon chip 130 rests on thebackplane and not the front plane, where the chip may interface withsurface roughness. Chip-to-polymer metal bonding technology using lowmelting temperature metal thin films provides flip-chip bonding.

The assembly is repeated across the skin 132 with additional circuitsthat handle multiple clusters for a distributed system. FIG. 23A showsan embedded sensor 139 and wiring 140 with an ASIC flip chip 130 bondedto backside vias 138 with solder bumps 142.

A second method, shown by example in FIG. 23B, includes thinning asemiconductor wafer 144 that contains analog/digital electronics at thetop surface 146 to the point that the semiconductor wafer becomesflexible and yet still maintains electronics functionalities. Forexample, a chip having a small die size (e.g., less than 1 cm²) withthickness on the order of 10-30 micrometers, may be used. An exemplarythinned silicon wafer is shown in FIG. 24. The silicon dies flex withthe polymer substrate 132 and therefore preserve the mechanicalflexibility. As shown in FIG. 23B, thin dies may be flip-chip bonded tobonding sites 148 on polymer sensor skin 132. The chip-to-polymerelectrical connection may be achieved, for example, using lowtemperature metal reflow. The top surface 146 can be further protectedand mechanically enhanced using conformal chemical vapor deposition of aplastic 150 such as Parylene, which is stress free, relatively soft, anddoes not damage the microelectronics or the sensor.

In a third method, shown by example in FIG. 23C and FIG. 25, bothcircuit elements 152 and sensor elements 154 are built on a siliconwafer 156 first. The sensors 154 are preferably formed on the wafer 156after the circuit elements 152 are formed (step (a) in FIG. 25). This isfeasible since the sensor elements 154 preferably can be formed underlow processing temperatures. An exemplary method uses a silicon wafer156 having preformed circuit elements, on which the sensor elements 154are formed. Such silicon wafers 156 may contain, for example, op-amps,multiplexors, and/or A/D conversion functions.

Post-process steps are performed to build interconnect wires 158 (step(b)) and the tactile or flow sensor elements 154. Next, the backside ofthe wafer 156 is patterned and etched (step (c)) to form trenches 160.An elastomer precursor 162 is poured and cured (step (d)), to encaseresulting silicon islands 164 in a elastomer back-filled skin. The frontsurface of the skin can be further protected, for example, by depositinga protective layer such as Parylene using chemical vapor deposition.These steps provide a flexible sensor chip 166, as shown flexed at step(e).

While specific embodiments of the present invention have been shown anddescribed, it is to be understood that other modifications,substitutions, and alternatives will be apparent to those of ordinaryskill in the art. Such modifications, substitutions, and alternativescan be made without departing from the spirit and scope of the presentinvention, which should be determined from the appended claims.

Various features of the invention are set forth in the appended claims.

1. A sensor chip comprising: a flexible, polymer-based substrate; atleast one microfabricated sensor disposed on said substrate andincluding a conductive element, said at least one sensor comprising atleast one of a tactile sensor and a flow sensor; a processor coupled tosaid at least one sensor for processing signals from said at least onesensor; wherein said processor is silicon based, and is embedded on aback surface of said substrate.
 2. A sensor chip comprising: a flexible,polymer-based substrate; at least one microfabricated sensor disposed onsaid substrate and including a conductive element, said at least onesensor comprising at least one of a tactile sensor and a flow sensor; aprocessor coupled to said at least one sensor for processing signalsfrom said at least one sensor; wherein said processor comprises asilicon die having a thickness equal to or lower than 50 μm, whereinsaid silicon die is flexible.
 3. The sensor chip of claim 2 wherein saidsilicon die is embedded on a front side of said substrate.
 4. Amicrofabricated hardness sensor comprising: a first contact areadisposed on a flexible membrane; a strain gauge for measuring a responseof the first contact area to an applied force from a contact object; asecond contact area disposed on a substrate, wherein said second contactarea is less flexible than said first contact area on the flexiblemembrane; another strain gauge for measuring a response of the secondcontact area to the applied force from the contact object.
 5. Themicrofabricated hardness sensor of claim 4 wherein the flexible membraneand the substrate on which the second contact area is disposed arepolymer-based, and wherein the flexible membrane is formed by removingmaterial from the polymer-based substrate.
 6. A microfabricated contactforce sensor comprising: a polymer-based flexible membrane having aplurality of flex channels formed on a back side therein; a plurality ofstrain gauges, each of said strain gauges formed on a front side of saidmembrane and over one of the flex channels, each of said strain gaugescomprising a conductive material for measuring a response of theflexible membrane to an applied force from a contact object.
 7. Amicrofabricated artificial haircell comprising: a cilium raised from asubstrate, the cilium being capable of deflection with respect to saidsubstrate; a strain gauge disposed on said cilium, which stretches orcompresses in response to an external force acting on said cilium. 8.The haircell of claim 7 wherein said strain gauge is disposed on a baseof said cilium, which is rigidly connected to said substrate via asupport.
 9. The haircell of claim 7 wherein said substrate is apolymer-based substrate.
 10. The haircell of claim 7 wherein said ciliumcomprises polyimide.
 11. The haircell of claim 7 wherein said cilium iscoupled to said substrate via a conductive hinge.
 12. The haircell ofclaim 11 wherein said conductive hinge is electroplated.
 13. Thehaircell claim 7 wherein said cilium is plated with permalloy.
 14. Amicrofabricated pressure and shear sress sensor comprising: a membranediaphragm; at least one thin-film conductive lead disposed on saidmembrane diaphragm, said conductive lead comprising a piezoresistor.